1. Field of the Invention
This invention relates to a two-color printing process, and more particularly, to a method for carrying out a two-color printing process within a thermal printer suitable for use in a point-of-sale environment.
2. Background Art
A thermal printer conventionally employs a single printhead to print on a paper web moved past the printhead. The printhead includes number of pixel-sized heating elements extending in a line across the width of the paper web, arranged to contact the paper web. The heating elements are individually electrically driven in a pattern used to produce the desired printed image.
Printing within a conventional thermal printer is by either a direct or a transfer process. In the direct process, the paper is coated with a material which changes color, typically turning black, upon the application of heat. (Black is herein referenced as a xe2x80x9ccolor,xe2x80x9d since it is formed by printing processes which can alternately be used to produced an individual color.) Thus, a black pattern is typically formed on the surface of the paper in accordance with the pattern in which the heating elements are driven as the paper is moved past the printhead.
On the other hand, in the transfer process, a film web, lying between the paper web and the printhead, is moved with the paper web. The film web includes a coating, lying next to the paper, which is locally softened and transferred to the paper in response to an application of heat. Thus, this coating, which again is typically black, is removed according to the pattern in which the heating elements are driven, to be transferred to the adjacent paper surface, forming the printed image.
The main advantage of the direct process is its simplicity compared to the transfer process. The main advantage of the transfer process is that it produces a document on plain paper, having a better surface xe2x80x9cfeelxe2x80x9d and being insensitive to heat. The main disadvantage of the transfer process is its cost, since the additional film ribbon web is more expensive than the coated paper. Also the web, which is rapidly used up, since it must move with the paper during printing, becomes another item to be stocked, replaced, and disposed during operation of the printer.
Because of its relative simplicity and low cost, the direct process is typically used for point-of-sale printers to print receipts and other documents rapidly and quietly, without the noise typically associated with impact printers in this application. At the point of sale, frequent replacement of supplies, such as film web, under the conditions of busy sales terminals and waiting lines, would be considered a serious disadvantage of the conventional transfer process. Additionally, the relatively small size of most receipts tends to eliminate objections to physical characteristics of the coated paper. Because documents received from facsimile transmissions are generally the size and shape of letter-size paper, because such documents are often filed with other documents of similar size and shape, and because of a longstanding level of general dissatisfaction with coated and tightly rolled paper for printing facsimile transmissions, the use of the transfer process with plain, letter-size paper is gaining wide acceptance in facsimile machines.
In the point-of-sale environment, what is needed is a method for using color to highlight specific information on a sales receipt, such as the total or amount due, so that such information can quickly be found in a list of numbers. Additionally, other information, such as advertising slogans or logos, printed on receipts may advantageously be highlighted using color. One method for providing such information highlighting involves returning to the conventional impact printer with a two color (typically red and black) ribbon. However, this method requires relinquishing the advantages of thermal printing in this environment, such as quiet operation, improved reliability with fewer moving parts, and a print quality that does not deteriorate over the life or a conventional ink ribbon.
In thermal printing, both the direct process and the transfer process can be modified to produce multicolor printed images. For example, U.S. Pat. No. 6,140,513 describes a recording material for use in the direct process of a two-color forming heat-sensitive system. The recording material has a single recording layer that forms at least two colors with a single kind of dye precursor experiencing changes in its chemical structure according to a level of heat energy applied to the recording layer.
A printer using the thermal transfer process to produce two or more colors is described in U.S. Pat. No. 4,672,393. The transfer film is formed as a ribbon having two or more layers, each of which has a different color and a different melting point, with the outermost layer, which is moved adjacent the recording paper on which printing is to occur, having the lowest melting point, and with the innermost layer, which lies against the film base of the ribbon, having the highest melting point. Thus, the number of layers transferred to the recording paper, and hence the color of the image formed by printing, is varied by changing the temperature of the heating elements within the printhead of the printer.
The principle disadvantage of each of these methods lies in the cost of its associated materials when compared to the single-color direct printing process. A direct printing thermal paper producing two colors is known to be substantially more expensive than conventional, single-color direct thermal printing papers. Even the single-color transfer process is known to require more expensive materials than the single-color direct printing process, and to require the disruption of frequent replacement of the thermal printing film cartridge. Therefore, in the point-of-sale environment, what is needed is a thermal printer capable of providing color highlighting, with the vast majority of the material being printed in black, without providing a substantial penalty in terms of materials cost or in terms of a frequent need to replace materials, due to the relatively infrequent use of a highlighting color.
The patent literature also includes descriptions of several thermal printing processes which print the subtractive primary colors, yellow, cyan, and magenta, in a manner causing their combination to represent any color. Such methods are too complex, expensive, and slow to be competitive for printing receipts in the point-of-sale environment. For example, U.S. Pat. No. 5,247,313 describes a direct thermal process in which the three layers are printed sequentially, with each layer being fixed to prevent further printing of its color after it is printed. U.S. Pat. Nos. 4,250,511 and 5,266,272 describe transfer thermal processes in which using a transfer film coated with transversely extending strips of different colors. In the process of U.S. Pat. No. 4,250,511, the transfer film is moved at a rate fast enough to allow each of the colors to be printed in a line of pixels before the paper moves off the line of pixels. In the process of U.S. Pat. No. 5,266,272, the paper is moved back and forth to print the different color layers sequentially from the different color layers of the transfer film.
According to a first aspect of the invention, apparatus is provided for direct thermal printing on a paper web having a first thermally sensitive coating turning from a light condition to a darkened condition at temperatures above a first temperature, with the apparatus additionally providing for transfer thermal printing on the paper web. The apparatus includes a thermal printhead, a paper drive motor, a transfer film web, a film drive motor, and a number of heating elements extending within the thermal printhead. The paper drive motor moves the paper web in a first direction past the thermal printhead and in proximity with the printhead, with the first thermally sensitive coating facing the thermal printhead. The transfer film web extends between the thermal printhead and the paper web and includes a second thermally sensitive coating facing the paper web. The film drive motor moves the transfer film web in a second direction between the thermal printhead and the paper web. The heating elements are individually operable to produce a pattern of heated and darkened areas on the first thermally sensitive coating of the paper web and to transfer a pattern of material from the second thermally sensitive to the paper web from the transfer film web as the paper web is moved in proximity to the thermal printhead.
Preferably, the material from the thermally sensitive coating transferred to the paper web includes a material transmitting ambient light to a surface of the paper web and transmitting light reflected from the paper web outward while absorbing light outside a range of frequencies.
Preferably, the apparatus additionally includes control logic, causing the paper drive motor to be operated and elements within the plurality of heating elements to be driven to turn the first layer to the darkened condition to print a darkened pattern on the paper web, and also causing the paper drive motor and the film drive motor to be operated and elements within the plurality of heating elements to be driven to transfer the material from the second thermally sensitive coating to the paper web at a temperature not driving the first thermally sensitive material into the darkened condition to print a color pattern on the paper web.
The apparatus may also include a timer operating during operation of the paper drive motor and reset by operation of the film drive motor, wherein the first timer causes operation of the film drive motor following a predetermined interval of operation of the paper drive motor without operation of the film drive motor.
According to another aspect of the invention, a method is provided for printing black and color patterns on a paper web having a thermally sensitive direct printing layer darkening when exposed to a first temperature. The method includes holding the paper web against a transfer film web, having a thermally sensitive transfer layer, wherein the thermally sensitive direct printing and transfer layers are disposed adjacent one another; applying heat in a first pattern to a side of the transfer film web opposite the paper web to produce a black pattern of darkened material within the thermally sensitive direct printing layer; and applying heat in a second pattern to the side of the transfer film web opposite the paper web to cause portions of the thermally sensitive transfer layer to transfer from the transfer film web to the paper web, wherein the heat applied in the second pattern is insufficient to produce a black pattern of darken material within the thermally sensitive direct printing layer.